It is known that sinterable piezoelectric ceramic material and piezoelectric composite material may be used for such towed piezoelectric cable.
Generally, piezoelectric ceramic-polymer composite can be produced by mixing ferroelectric ceramic particles of lead zirco-titanate or lead titanate with piezoelectric organic material such as polyvinylidene fluoride, polyvinyl fluoride, polyvinylidene chloride, polyvinyl chloride or nylon, or organic material such as synthetic rubber or synthetic resin, and has acoustic impedance characteristics similar to that of water.
It is also known that when such a piezoelectric ceramic-polymer composite material is used for a piezoelectric cable, it can efficiently receive acoustic waves being propagated under water to provide advantages of increasing the sensitivity.
U.S. Pat. No. 3,860,899 discloses a towed hydrophone system for sensing acoustic signals in which it comprises a towed cable, a hydrophone housing secured at one end of the towed cable and filled with a viscous fluid, and a hydrophone positioned axially in the housing by an axially extending conductive connector.
U.S. Pat. No. 3,798,474 discloses a piezoelectric sensor system which comprises at least one element of flexible material having piezoelectric properties and two faces, each associated with an electrode. This U.S. Pat. No. 3,798,474 also discloses a sensor of a coaxial shape comprising a sensitive piezoelectric element having two cylindrical electrodes placed respectively inwardly and outwardly with respect to the piezoelectric element, a center core of flexible insulating material for supporting the assembly, and a flexible insulating sheath.
Further, U.S. Pat. No. 4,183,010 discloses a coaxial piezoelectric cable comprising radially spaced conductors, intermediate dielectric material having piezoelectric properties, and an outer jacket.
With the conventional arrangement as mentioned above, the piezoelectric cable is subjected to not only acoustic waves but also a tensile stress or a bending stress due to nodding when it is towed. By these mechanical pressures there may be produced some distortion in the piezoelectric element to induce an electric charge or voltage which produces a noise signal to be superposed on the acoustic waves, thereby decreasing the S/N ratio.
Further, when the piezoelectric cable is towed, there may occur a cavitation or turbulence around the cable which produces an undesired signal called a flow noise. In case the piezoelectric element assemblies are disposed with leaving a larger space therebetween, said flow noise can not be satisfactorily suppressed.
Furthermore, when the coaxial cable is subjected to a force in a direction perpendicular to the axis thereof, it may be deformed to have the depressed portion and the protruded portion which are different from each other in area. Therefore, the produced noise signals can not be cancelled to each other. Also, for the axial tensile stress a noise signal may be introduced into an acoustic signal to be determined.
In the prior art as mentioned above, the influence of the various mechanical pressures which act on the piezoelectric cable can not fully eliminated, so that some noise signals are necessarily introduced into the measured acoustic signal and thus a high S/N ratio can not be obtained.